1. Field of the Invention
The present invention relates to an amplifier circuit.
2. Description of the Related Art
In wireless communications systems such as mobile phones or wireless data communications devices, an amplifier circuit for amplifying a received signal is provided on a receiving side. An example of an amplifier circuit may include, for example, a low noise amplifier (LNA). An LNA is an amplifier circuit that amplifies a signal by reducing noise generated in the circuit by as much as possible and is an essential circuit disposed at a front end of a wireless receiving circuit.
As described above, as demand for inexpensive LNAs has increased, LNAs have been formed of a complementary metal oxide semiconductor (CMOS).
In addition, since the case in which a transmitter is adjacent to a receiver may exist, an over-input signal, input to the LNA, may be generated. However, a gate oxide film of an input transistor in the LNA may be deteriorated or damaged by the over-input signal. Therefore, it is important to address the issue of the over-input signal. An example of an over-input signal counter-measure may include a method of using a MOS transistor with high pressure resistance in an input of the LNA. In addition, Patent Document 1 discloses a protective circuit for increasing resistance against a surge voltage with a complementary diode formed by reversely connecting diode-connected MOS transistors in parallel.
However, the method of using the MOS transistor with the high pressure resistance at the input of the LNA may increase a gate length and an oxide film thickness of the MOS transistor. For this reason, a cutoff frequency of the MOS transistor may be deteriorated and as a result, a noise figure (NF), an important characteristic of the LNA, may be deteriorated. In addition, in the protective circuit disclosed in Patent Document 1, one end of the complementary diode is connected to a ground (GND) and a function of adjusting an upper limit voltage or a lower limit voltage of the input is not furnished.